Field emission electron gun

ABSTRACT

A field emission electron gun comprises a field emission tip as its source of electrons. A first anode is spaced downstream from the tip and when a voltage is applied between the first anode and the tip, electrons from the tip are accelerated toward the first anode. An opening in the first anode limits the angular spread of the electron beam. A second anode is spaced downstream from the first anode and when a voltage is applied between the second anode and the tip, the energy level of the electrons at the image or specimen plane is controlled. The electrostatic field between the first and the second anode brings the electron beam into focus. For protecting the field emission tip against high voltage discharges, a third electrode in the form of a shield surrounds the field emission tip and is maintained at or near the electrical potential of the tip. Within the shield is a fourth electrode which serves, when voltage is applied thereto, to draw electrons from the tip and to restore or maintain normal operating conditions for the field emission electron gun. An iongetter vacuum pump and a reactive sublimator vacuum pump are formed in the electron gun by evaporating a highly reactive element or getter material on the inner walls of the third electrode, which serves as a collector by inducing gas molecules which strike this surface to adhere thereto and to be imbedded therein. The inner walls of the third electrode react with reactive gasses present in the region of the tip and the fourth electrode. The ion getter pump operates by ionizing residual gas molecules which are then impelled by electric fields and are imbedded under the coating of sublimed getter material. The primary electron beam from the tip strikes the surface of the fourth electrode, thereby causing reflected and secondary electrons to be emitted from the surface, which electrons form an electron cloud capable of ionizing molecules within the chamber. The electron cloud is formed and the ionized gas molecules are collected by applying the appropriate potentials to the electrodes in the gun assembly. The third electrode may be cooled by a liquid nitrogen cooling system, which functions as a cryogenic vacuum pump. This cooling system can also be used to cool the tip in order to reduce the tip flicker noise resulting in greater stability of electron emission.

United States Patent 91 Coates et al.

[111 3,766,427 [451 Oct. 16, 1973 FIELD EMISSION ELECTRON GUN [73] Assignee: American Optical Corporation,

Framingham, Mass.

[22] Filed: Feb. 7, 1972 [21] Appl. No.: 224,362

Related US. Application Data [63] Continuation of Ser. No. 46,425, June 15, 1970, Pat.

[52] US. Cl. 315/31 R, 250/49.5 C, 250/49.5 R, 250/49.5 GC, 250/495 A OTHER PUBLICATIONS Crewe, Electron Gun Using Field Emission Source, Review of Scientific lns., Vol. 39, No. 4.

Primary Examiner-Carl D. Quarforth Assistant Examiner.l. M. Potenza Att0rneyWilliam C. Nealon et al.

[57] ABSTRACT A field emission electron gun comprises a field emission tip as its source of electrons. A first anode is spaced downstream from the tip and when a voltage is applied between the first anode and the tip, electrons from the tip are accelerated toward the first anode. An opening in the first anode limits the angular spread of the electron beam. A second anode is spaced downstream from the first anode and when a voltage is applied between the second anode and the tip, the energy level of the electrons at the image or specimen plane is controlled. The electrostatic field between the first and the second anode brings the electron beam into focus.

For protecting the field emission tip against high voltage discharges, a third electrode in the form of a shield surrounds the field emission tip and is maintained at or near the electrical potential of the tip. Within the shield is a fourth electrode which serves, when voltage is applied thereto, to draw electrons from the tip and to restore or maintain normal operating conditions for the field emission electron gun. An ion-getter vacuum pump and a reactive sublimator vacuum pump are formed in the electron gun by evaporating a highly reactiveelement or getter material on the inner walls of'the third electrode, which serves as a collector by inducing gas molecules which strike this surface to adhere thereto and to be imbedded therein. The inner walls of the third electrode react with reactive gasses present in the region of the tip and the fourth electrode. The ion getter pump operates by ionizing residual gas molecules which are then impelled by electric fields and are imbedded under the coating of sublimed getter material. The primary electron beam from the tip strikes the surface of the fourth electrode, thereby causing reflected and secondary electrons to be emitted from the surface, which electrons form an electron cloud capable of ionizing molecules within the chamber. The electron cloud is formed and the ionized gas molecules are collected by applying the appropriate potentials to the electrodes in the gun assembly. The third electrode may be cooled by a liquid nitrogen cooling system, which functions as a cryogenic vacuum pump. This cooling system can also be used to cool the tip in order to reduce the tip flicker noise resulting in greater stability of electron emission.

9 Claims, 8 Drawing Figures United States Patent 1 11 3,766,427 Coates et al. Oct. 16, 1973 IIII llllf lll lll PATENTED URI 16 i973 SHEET 1 UF 5 PATENTEDHBI 16 me. 3.7663127 SHEET 5 BF 5 FIELD EMISSION ELECTRON GUN This is a continuation, of application Ser. No. 46,425, filed June 15, 1970, now US. Pat. No. 3,678,333.

BACKGROUND OF THE INVENTION The present invention relates in general to electron optical systems, and more particularly to a field emission electron gun.

Electron microscopes have heretofore comprised a field emission gun which included a field emission tip as its source of electrons. Spaced downstream from the tip was a first anode. A voltage applied between the tip and the first anode drew electrons from the tip and accelerated the electrons. Generally, the applied voltage was in the order of 500-5,000 volts. A second anode was spaced downstream from the first anode and a voltage applied between the tip and the second anode further accelerated the electrons. Generally, the applied voltage was in order of l,000l00,000 volts.

High voltage discharges taking place within the field emission gun subjected the field emission tip to excessive potentials, which resulted in high tip current and subsequent melting of the field emission tip. As a consequence thereof, the field emission tip failed prematurely and required frequent replacement.

Field emission tips have been disposed within chambers at ultra high vacuums in the order of l Torr. or lower in order to operate with stability. Instability arises out of excessive gas molecules in the vicinity of the tip, which strike the tip or absorb to the tip surface to cause erratic electron emission. Vacuums in the order of lO-- Torr. or lower have been produced by expensive, complex pumping systems.

Disclosures of field emission electron guns of the type above-described are found in the article entitled Electron Gun Using a Field Emission Source by A. V. Crewe, D. N. Eggenburger, J. Wall, and L. M. Welter, published in The Review of Scientific Instruments, Volume 39, Number 4, April 1968, Pages 576583; US. Pat. to A. V. Crewe, No. 3,191,028, issued on June 22, 1965, for Scanning Electron Microscope; an article entitled A High-Resolution Scanning Electron Microscope by A. V. Crewe, J. Wall and l... M. Welter, published in the Journal of Applied Physics, Volume 39, No. 13, Pages 5861-5868, December 1968.

SUMMARY OF THE INVENTION A field emission electron gun for an electron optical system having a field emission tip as a source of electrons. A shield electrode surrounding the field emission tip is electrically connected to the field emission tip for protecting the field emission tip against voltage discharges.

A further feature is that another electrode is disposed within the shield electrode to draw electrons from the tip and to maintain normal operating conditions for the electrode to attract positive ions away from the tip. The electrons produced by the incident primary beam current from the tip at the electrode within the shield electrode are in a negative electric field, and, hence, tend to form a constrained, high density electron cloud within the shield electrode. The electron cloud increases the probability of ionizing gas molecules in the region contained by the shield electrode, and these ionized molecules are then attracted toward the shield electrode.

In the exemplary embodiment of the present invention, a reactive element or getter material is evaporated on the inner wall of the shield electrode to induce molecules impinging on the shield electrode to adhere thereto and to be imbedded in the inner wall of the shield electrode. In addition, when the beam of electrons is emitted from the tip, the inner wall of the shield electrode serves as an ion collector to attract and capture in the reactive coating those gas molecules which are ionized by the primary beam and by the electron cloud which the beam generates. Through this action, a getter-ion pump is present for producing and maintaining ultra high vacuums by ionizing gas molecules which are then impelled to the shield wall by electric fields and are buried there under the coating of sub- -limed getter material. A reactive sublimator pump is also present by inducing reactive gas molecules which happen to strike the shield electrode to stick there and to be buried under the getter material.

It has been found that the number of gas molecules liberated from the electrode within the shield electrode increases as the current emitted from the field emission tip increases. The number of secondary and reflected electrons generated at the surface of the electrode within the shield electrode also increases as the current emitted from the field emission tip increases. Therefore, the pumping efficiency of the present getter-ion pump increases as the pumping requirements increase. In addition, the shield electrode may be cooled to improve the overall pumping efficiency of the vacuum system. This improvement occurs because of the increased pumping for the condensable gases and the more efficient pumping of reactive gases-The same cooling system is used to cool the tip which reduces flicker noise and improves tip current stability.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 7 is a horizontal section view taken along line 7-7 of FIG. 4.

FIG. 8 is a horizontal section view taken along line 8-8 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Illustrated in FIG. 1 is an electron optical system in the form of a scanning electron microscope which embodies therein a field emission gun of the present invention. The field emission gun 15 produces from its electron source a bright, focused spot of electrons at its image plane to illuminate a specimen 20. The focused spot is scanned by means of a deflection system and stigmator 25. Information about the specimen is obtained by detecting transmitted electrons, secondary electrons, reflected electrons, absorbed electrons, photons, or X-rays, any or all of which are generated by the incident electron beam. Detectors 30 and 30a are often used to detect one of these signals which is then used to modulate the intensity of a synchronously scanned display tube 35 with a sweep generator 36 to form an image of the specimen 20.

In FIGS. 2-8 is illustrated the field emission gun 15 of the present invention, which comprises a substantially cylindrical housing 40 defining a vacuum chamber 41 (FIGS. 4 and 5). Disposed within the vacuum chamber 41 and the high vacuum area 41a thereof is a source of electrons, such as a field emission tip 45. The field emission tip 45 is an etched tip which does not employ any filament voltage or power to emit electrons. The cold field emission tip 45 is very small in diameter, such as 1,000A.

For supporting the field emission tip 45 for movement along the x, y and 2 planes and for the removal of the field emission tip 45 for replacement, a tip support assembly 50 comprises a V-shaped mount 51, which has the tip 45 attached thereto. The V-shaped mount 51 is crimped between electrodes 52 and 53 in fixed relation. An insulator disc 54 for the tip 45 receives the electrodes 52 and 53 and the electrodes 52 and 53 are secured to the insulator disc 54 by nuts 56 and 57. The electrodes 52 and 53 are received as prongs by the lower end of a high voltage insulator post 55. Nuts 56 and 57 on the electrodes 52 and 53 limit the extent of the entry of the electrodes 52 and 53 into the distal end of the post 55. From this arrangement, the tip 45, the disc 54 and the electrodes 52 and 53 are in the form of a plug which is detachably secured to the distal end of the post 55. For replacing the tip 45, the plug is detached as a unit and replaced as a unit.

The upper end of the insulator post 55 is fixedly secured to a metal shaft 60 having an end cap 60a with a depending flange fixed within a tapped hole of the upper end of the post 55. Mounted on the cap 60a in fixed relation is a bellows 61. The bellows 61 are welded to the cap 60a to form a vacuum wall or seal with the high vacuum chamber while permitting axial and transverse movement of the tip support assembly 50. The bellows 61 are welded directly to an end cap 62 to complete the vacuum seal. Surrounding the shaft 60 is a sleeve 63 (FIG. 4) that is received with the shaft 60 by an opening 64 in the cap 62. Spaced from the cap 62 is a support plate 65 that has a suitable opening therein for receiving the shaft 60 and the sleeve 63.

To tilt the plate 65 relative to the cap 62 in two different planes, screws 66 engage the cap 62 at their free ends and are in threaded engagement with the plate 65. This produces x and y motion of the tip by pivoting sleeve 63 on pivot bearing 67 fixed to the cap 62 by a pivot bearing mount. Thus, as sleeve 63 pivots, so does shaft and tip support assembly 50 resulting in x and y motion of the tip. Springs, not shown, cooperate with the screws 66 and 67 to maintain the plate in its adjusted position relative to the cap 62. A knob 68 is in threaded engagement with the upper end of the shaft 60 and is rotatable at a fixed height. Thrust bearings 69 permit the rotation of the knob 68 relative to the plate 65. Thus, rotation of the knob 68 imparts axial or vertical movement to the shaft 60 without rotating the shaft 60. Vertical movement of the shaft 60 imparts a vertical movement to the tip 45 through the post 55 and the electrodes 51 and 53. Movement of the tip 45 in the vertical direction serves as a means for focusing the electron beam emitted from the tip 45.

Spaced downstream from the tip 45 in the low vacuum area 41b of the vacuum chamber 41 is a first anode of a first focusing electrode formed with an outer cylindrical wall and a central opening 84. The first anode 75 is well-known and is described in the aforementioned article by A. V. Crewe et al. entitled Electron Gun Using A Field Emission Source."

Spaced downstream from the first electrode 75 is a second anode or second focusing electrode 85, which has an annular cylindrical wall and an axial cylindrical wall. An annular insulating support 86 with a stepped inwardly facing configuration supports the first anode 75 on the shoulder thereof and is fixed to the second anode to be supported thereby.

When a positive voltage V is applied between the tip 45 and the second anode 85, the electrons emitted from the tip 45 are further accelerated. In the typical embodiment, the voltage V is equal to 20,000 volts. The second anode is attached to the chamber of the gun and in turn to the more positive terminal of the V, supply. The second anode 85 is well-known and is described in the aforementioned article by A. V. Crewe et al. entitled Electron Gun Using A Field Emission Source. It is the second anode 85 that controls the energy level of the electrons impinging on the specimen 20.

It has been found that the tip 45 fails, when subjected to excessive discharge voltages, because of the high tip current which then flows and melts the end of the tip. According to the present invention, a third electrode or shield electrode is disposed within the high vacuum area 41a of the vacuum chamber 41 to reduce premature failure of the tip 45 and to extend the life thereof.

The shield electrode 90 has an outer cylindrical wall with an upper end wall of a reduced diameter opening and with an inwardly turned, arcuate central wall. As shown in the drawings, the shield electrode 90 surrounds the tip 45 and is connected to an output terminal 77 of the insulator 78 (FIG. 4). Thus, the third electrode 90 is substantially at the same electrical potential as is the tip 45 and is electrically connected thereto through a low impedance. It is the shield electrode 90 that protects the tip 45 from excessive voltage discharges from the first and second anodes, and, hence, the tip 45 is not exposed to high voltage transients. As a consequence thereof, the life of the tip 45 is extended. In the exemplary embodiment, the shield electrode 90 is made of mu metal, which provides a shield against stray outside magnetic fields that tend to deflect the electron beam emitted by the tip 45.

A fourth electrode is disposed within the shield electrode 90. When a voltage is applied to the fourth electrode 95, electrons are drawn from the tip 45. It is the fourth electrode 95 that restores the electron emission gun to normal operation or maintains the operation of the electron gun normal, when a shield electrode is employed to protect the tip 45 from excessive voltage transients. The source of high voltage, herein referred to as V,, is in the typical embodiment in the order of a positive 500 -3,000 volts. When the voltage V, is applied between the tip 45 and the fourth electrode 95, electrons are drawn from the tip 45 and are accelerated toward the first anode 75. The fourth electrode 90 is connected to the first anode through a current limiting resistor.

For this purpose, a suitable high voltage supply 76 has its low voltage output connected to a terminal 77 of the high voltage insulator 78. The terminal 77 is also connected to the electrode 52, of the tip mounting assembly 50 over conductor 81 (FIG. 7) as well as to the shield electrode 90. A terminal 77' is connected to the electrode 53 over the conductor 80. The terminal 79, which is the high voltage V, output, is connected over a conductor 82 to the fourth electrode 95 and, in turn, is connected to the first anode 75 through a voltage dropping resistor 83 (FIG. 4). This voltage dropping resistor 83 serves to maintain the fourth electrode 95 at normal V, voltage when a high voltage discharge causes the first anode 75 to are to the second anode 85. The beam of electrons eminating from the tip 45 passes through a small opening in the fourth electrode 95, which is narrower than the beam of electrons and thereby controls the angular spread of the beam of electrons passing on to the first anode 75 and through its opening 84. Note that the electrons that are incident on the fourth electrode 95 and generate the electron cloud, initiate the pumping action of the getter-ion of the shield electrode 90.

It is to be observed that the assembly for adjusting the location of the tip 45, which has movement in the x, y and z planes, is separate and apart from the high voltage insulator 78. When the high voltage insulator established its connections through the tip support assembly 50, the tip 45 was subjected to vibrations. Vibrations on the tip 45 tend to cause degradation in the resolution of the scanning electron microscope 10. Thus, permanent connections are established with the electrodes 52 and 53 through the flexible conductors 80 and 81, yet enabling the tip 45 to be moved within the vacuum chamber 41 in the x, y and z planes.

It is known that excess gas molecules in the vicinity of the tip 45 cause erratic or noisy electron emission from the tip 45. This results in poor resolution for a scanning electron microscope. Thus, it was customary to employ powerful and expensive pumping systems to obtain a high vacuum area for the field emission tip.

According to the present invention, a very reactive element, or getter material, such as titanium, is evaporated from an annular filament 91 onto the inner wall of the shield electrode 90. One end of the filament 91 is connected to the terminal 77 and the other end of the filament 91 is connected to the voltage 62. The evaporation of the titanium on the inner wall of the shield electrode 90 may be either a continuous process or a periodic process. The titanium on the wall of the shield electrode 90 will react with and imbed therein reactive gasses that arrive at its surface. It therefore helps produce ultra high vacuums by collecting and burying gas molecules under its coating of sublimed getter material.

However, electrons emitted from the tip. are accelerated toward and strike the fourth electrode 95 causing large local pressure increases by liberation of positive ions, negative ions'and neutral molecules from the surface of the fourth electrode 95. In addition, secondary electrons, reflected electrons and energetic X-rays are emitted. The liberation of the molecular species plus molecules which enter from other places would cause the tip to contaminate and the electron emission from the tip to become unstable. Such a condition would impair the operation of the electron emission gun by making it noisy, erratic and subject to premature failure. Further, instability maybe caused by some positive ions which are accelerated back to the field emission tip and damage it.

The shield electrode of the present invention is held at an electrostatic potential which attracts these positive ions away from the tip 45 as an ion collector. The secondary and reflected electrons produced at the surface of the fourth electrode are within a negative electrostatic field and tend to form a high density electron cloud confined within the shield of the third electrode 90. The electron cloud increases the probability of ionizing any gas molecules in the high vacuum area which are then attracted toward the shield electrode 90. Thus, a local getter-ion pump operation is established in conjunction with the sublimation pumping.

Although the above-described operation can be per formed with the shield electrode 90 at ambient temper atures, the pumping efficiency of the molecules in the titanium covered shield electrode -90'can be improved by cooling the shield electrode 90. Toward this end, the shield electrode 90 is cooled by a liquid nitrogen cooling system 100. The cooling system comprises a container 101 of toroidal configuration for storing liquid nitrogen, which container 101 isconnected to the shield electrode 90 through a beryllium oxide insulator 102 (FIG. 4) also of an annular configuration. The beryllium oxide insulator 102 is not only a good high voltage insulator but also has a high thermal conductance. The shield electrode 90 because of its strong negative electrostatic field not only produces theielectron cloud of secondary electrons but also holds back the secondary electrons from being accelerated onto the inner wall of the housing 40, which defines the vacuum chamber 41, thus causing further outgassing and pressure increases. Therefore, a cryogenic pump operation is performed in the high vacuum area 41a of the vacuum chamber 41. I

Through the getter-ion pump operation of the shield electrode 90 and the cryogenic pump operation of the nitrogen cooling system 100, a high vacuum in the order of 10- Torr. is created in the vacuum chamber 41 without expensive, independent vacuum pumps.

Electrons emitted from the tip 45 travel through the opening in the fourth electrode 95, through the opening 84 of the first electrode 75, through a similar opening 106 in the second electrode 85, through a beam guide tube 107 disposed axial of the deflection system and stigmator 25 to impinge on the specimen 20. For controlling the cross-sectional area of the electron beam, an aperture selector plate 110 is disposed between the second anode 95 and the deflecting system and stigmator 25. The aperture selector plate 110 includes a series of openings varying in diameter. In the exemplary embodiment, there are four such holes ranging in diameter from 25 to 250 microns.

By sliding the plate 110 horizontally, a selected aperture is aligned with the tube 107 and the opening 106 of the second anode 85 to control the size of the electron beam advancing therethrough. For this purpose, a shaft 111 is secured to the plate 110 for imparting a horizontal movement thereto. A housing 112 is fixed in sealing engagement with the housing 40 through rings 113. O-rings 114 provide a seal between the shaft 111 and the housing 112. Seated in the housing 114 and surrounding the shaft 111 are bellows 115, which are welded thereto to permit the shaft 1 1 1 to have axial movement while maintaining a fluid seal. Outside the housing 40, the shaft 111 is threaded to knobs 116 and 117. Suitable bearings 118 enable the knobs 116 and 1 17 to be rotated relative to a cap 120 for the housing 112. Thus, rotation of the knob 117 imparts reciprocal movement to the shaft 111 without rotating the shaft 111 to align a selective opening of the plate 111 with the path of travel of the electron beam to control the diameter thereof. Knob 116 is an eccentric that imparts transverse motion to the plate 110 about the O-ring 1 14 which serves as a pivot point for the transverse motion.

When the tip 45 is changed, the chamber 41 is at atmospheric pressure. To facilitate the reduction of pressure in the chamber 41 from atmospheric to a high vacuum, the chamber 41 is exposed to the atmosphere by fully retracting the shaft 111 through the knob 117. When this is done, a vent 125 in the housing 112 communicates through an opening 126 with the chamber 41 by the O-rings 114 adavancing into the increased diameter portion of the housing 112. By returning the shaft 111 to its initial position, the valve is closed and the chamber 41 is not vented through the opening 125, since the O-rings 114 occupy the position shown in FIG. 4. Differential pumping occurs between the high vacuum in the chamber 41a and the chamber 41b through the opening 84 on the first anode 75.

For isolating the chamber 41 from atmospheric pressure while the specimen 20 is being changed, a valve 130 (FIG. 4) with an opening 131 formed in a valve shaft 143 is slidable in a horizontal direction so as to remove the opening 131 from alignment with the lower end of the tube 107 to block the same. O-ring 143' seals the end of the opening 107 when the valve 130 is closed. After the specimen is changed, the opening 131 is realigned with the bottom of the tube 107. A suitable vacuum pump 135 is attached to the lower wall of the emission gun and draws out the air under atmospheric pressure in the chamber 41 until the chamber is under a vacuum in the order of 10- or l0 Torr.

For sliding the valve 130 into and out of venting position, a valve housing 136 is fixed to the housing 40. The housing 136 is sealed by a cap 137 and O-rings 138. 0- rings 139 provide a seal between the housing 136 and the housing 40. Bellows 141 welded to the cap 137 and a sleeve 142 surrounding the valve shaft 143 permit rectilinear motion of the valve shaft 143 without breaking the seal. A knob 145 in threaded engagement with the valve shaft 143 is rotated to impart a rectilinear movement to the shaft 143 without rotating the same.

After the chamber 41 has been evacuated to about lO-- or 10- Tom, the shaft 111 is moved to close off the vent 125, thereby closing the differential valve and the opening 131 is aligned with the tube 107 through the movement of the valve shaft 143. When the opening 131 is aligned with the tube 107, electrons are permitted to impinge on the specimen 20. At this time, the vacuum in the chamber 41 is maintained by the cryogenic pump 100, the titanium sublimator and ion getter pump 90, and the vacuum in chamber 41b is maintained by the pump 135 at about 1O Torr. These pumps are adequate to evacuate the high vacuum chamber to 10 Torr. This vacuum provides stable operating conditions for the tip 45. The differential pumping now takes place through the opening 84 of the first anode 75, since this is the only path for establishing communications between the high vacuum chamber 41a and a low vacuum chamber 41b below the opening 84.

In changing a specimen, the valve shaft 143 is employed to isolate the high vacuum chamber 41a of the field emission gun 15 keeping it at a high vacuum, while the low vacuum chamber 41b is pressurized to atmospheric pressure.

The shield electric has its cylindrical wall connected to its base by suitable means such as a hinge contact 151. In addition thereto, the housing 40 has separable sections in its cylindrical wall at the O-rings 113, which are suitably connected through a suitable hinge 152. When it is desired to replace the tip 45, the gun 15 is pivoted at the hinges 151 and 152 to provide access for the replacement of the plug mounting for the emission tip 45.

We claim:

1. A field emission gun comprising:

a housing defining a vacuum chamber;

a field emission tip disposed in said vacuum chamber for providing a source of charged particles;

a first electrode disposed in said chamber and spaced downstream of said field emission tip;

a second electrode farther downstream of said field emission tip, said first electrode and said second electrode establishing focus and acceleration fields for forming a beam of said charged particles;

an intermediate electrode located mediate of said field emission tip and said first electrode for developing an electric field to generate said charged particles; and

voltage means connected to said first and second electrodes, said field emission tip, and said intermediate electrode for supplying electrical potential to establish said focus, acceleration and particle generation fields.

2. The field emission gun of claim 1 wherein said charged particles are electrons, and said potentials of said first, second and intermediate electrodes are posi tive with respect to said field emission tip.

3. The field emission gun of claim 1 wherein a shield electrode disposed in said vacuum chamber defines a region in which said field emission tip is disposed for preventing excessive voltage discharges to said field emission tip, and said intermediate electrode is operatively effective within said defined region to generate said charged particles.

4. The field emission gun of claim 3 wherein said voltage means includes impedance means interconnecting said intermediate electrode and said first electrode for limiting said intermediate electrode potential with respect to said field emission tip during discharge conditions from said second electrode to said first electrode, and said intermediate, first and second electrodes have centrally located apertures axially aligned with said field emission tip for passage of said charged particles forming said beam.

5. The field emission gun of claim 2 including means operatively associated with said intermediate electrode means and said field emission tip for vacuum pumping an area contiguous with said field emission tip.

6. The field emission gun of claim 5 wherein said field emission tip is disposed within a region defined by a shield electrode, and said means for vacuum pumping said area includes electrons from said field emission tip, and secondary and reflected electrons which ionize contamination particles, and means for capturing said ionized particles.

7. The field emission gun of claim 6 wherein a getter material is located within said area for capturing said contamination particles, and said ionized particles are attracted to said shield electrode.

8. The field emission gun of claim 1 including a second contiguous vacuum chamber for housing specimens upon which said charged particles beam impinges, said chamber and said specimen chamber being interconnected by at least one orifice of area selected so as to permit maintenance of said chambers at different vacuum levels. r

9. The field emission gun of claim 6 wherein said chambers are interconnected by at least one orifice through which said charged particle beam passes, said charged particles are electrons, and said chamber is thereby maintained at a substantially lower pressure than said specimen chamber.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N 3,766,427 Dated October 16, 1973 lnventofls.) Vincent J. Coates and Leonard M. Welter It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the Patent front page, delete data element identifier L6j and substitute:

463/ Division of Bar. No. 46,425,

I .June 15, 1970, Pat. No. 3,678,333.

In column; 1, line 2 delete "continuation" 'and substitute therefor --diviaion--.

Signed arid sealed this 6th day of August 1974.

(SEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-105O (10-69) USCOMM-DC GOING-P69 o .1.5. eovznmazm PRINTING OFFICE I90 D366-33|.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION pa e N 3,766,427 I Dated October 16, 1973 lnventoz-(s) Vincent J, Coates and Leonard M. Welter It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the Patent front page, delete data element identifier L/ and substitute:

L627 Division of Ser. No. 46,425,

' June 15, 1970, Pat. No. 3,678,333.

In column 1, line 2 delete "continuation" and substitute therefor --division--.

Signed a1 1d sealed this 6th day of August 1974.

(SEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN- Attesting Officer Commissioner of Patents F ORM PO-1050 (10-69) USCOMM-DC 00376-P69 if US. GOVERNMENT PRINY NG OFFICE 2 IQD 0-36643, 

1. A field emission gun comprising: a housing defining a vacuum chamber; a field emission tip disposed in said vacuum chamber for providing a source of charged particles; a first electrode disposed in said chamber and spaced downstream of said field emission tip; a second electrode farther downstream of said field emission tip, said first electrode and said second electrode establishing focus and acceleration fields for forming a beam of said charged particles; an intermediate electrode located mediate of said field emission tip and said first electrode for developing an electric field to generate said charged particles; and voltage means connected to said first and second electrodes, said field emission tip, and said intermediate electrode for supplying electrical potential to establish said focus, acceleration and particle generation fields.
 2. The field emission gun of claim 1 wherein said charged particles are electrons, and said potentials of said first, second and intermediate electrodes are positive with respect to said field emission tip.
 3. The field emission gun of claim 1 wherein a shield electrode disposed in said vacuum chamber defines a region in which said field emission tip is disposed for preventing excessive voltage discharges to said field emission tip, and said intermediate electrode is operatively effective within said defined region to generate said charged particles.
 4. The field emission gun of claim 3 wherein said voltage means includes impedance means interconnecting said intermediate electrode and said first electrode for limiting said intermediate electrode potential with respect to said field emission tip during discharge conditions from said second electrode to said first electrode, and said intermediate, first and second electrodes have centrally located apertures axially aligned with said field emission tip for passage of said charged particles forming said beam.
 5. The field emission gun of claim 2 including means operatively associated with said intermediate electrode means and said field emission tip for vacuum pumping an area contiguous with said field emission tip.
 6. The field emission gun of claim 5 wherein said field emission tip is disposed within a region defined by a shield electrode, and said means for vacuum pumping said area includes electrons from said field emission tip, and secondary and reflected electrons which ionize contamination particles, and means for capturing said ionized particles.
 7. The field emission gun of claim 6 wherein a getter material is located within said area for capturing said contamination particles, and said ionized particles are attracted to said shield electrode.
 8. The field emission gun of claim 1 including a second contiguous vacuum chamber for housing specimens upon which said charged particles beam impinges, said chamber and said specimen chamber being interconnected by at least one orifice of area selected so as to permit maintenance of said chambers at different vacuum levels.
 9. The field emission gun of claim 6 wherein said chambers are interconnected by at least one orifice through which said charged particle beam passes, said charged particles are electrons, and said chamber is thereby maintained at a substantially lower pressure than said specimen chamber. 